Debugging PHP code is part of any project, but WordPress comes with specific debug systems designed to simplify the process as well as standardize code across the core, plugins and themes. This page describes the various debugging tools in WordPress and how to be more productive in your coding as well as increasing the overall quality and interoperativity of your code.
WP_DEBUG is a PHP constant (a permanent global variable) that can be used to trigger the "debug" mode throughout WordPress. It is assumed to be false by default and is usually set to true in the wp-config.php file on development copies of WordPress.
define('WP_DEBUG', true); define('WP_DEBUG', false);
WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).
WP_DEBUG_DISPLAY is another companion to WP_DEBUG that controls whether debug messages are shown inside the HTML of pages or not. The default is 'true' which shows errors and warnings as they are generated. Setting this to false will hide all errors. This should be used in conjunction with WP_DEBUG_LOG so that errors can be reviewed later.
The SAVEQUERIES definition saves the database queries to an array and that array can be displayed to help analyze those queries. The constant defined as true causes each query to be saved, how long that query took to execute, and what function called it.
The array is stored in the global $wpdb->queries.
Example wp-config.php for Debugging
The following code, inserted in your wp-config.php file, will log all errors notices and warnings to a file called debug.log in the wp-content directory. It will also hide the errors so they do not interrupt page generation.
// Enable WP_DEBUG mode define('WP_DEBUG', true); // Enable Debug logging to the /wp-content/debug.log file define('WP_DEBUG_LOG', true); // Disable display of errors and warnings define('WP_DEBUG_DISPLAY', false); @ini_set('display_errors',0); // Use dev versions of core JS and CSS files (only needed if you are modifying these core files) define('SCRIPT_DEBUG', true);